The concentration of oxonium ions in solution can be expressed in terms of the pH scale.

The pH of a solution is the logarithm to base 10 of the reciprocal of the numerical value of the oxonium ion concentration:

pH = lg 1/H₃0⁺

= -lg [H₃0⁺]

= -lg [H⁺]

The pH of a neutral solution can be calculated directly from the ionic product of water.

Kw = [H₃O⁺] = [OH^-] OR [H⁺] = [OH^-]

[H⁺] = 10^-7

pH = 7 for neutral solutions.

Acidic solutions have a pH less than 7 at 25^oC.

The concentration of hydroxide ions in a solution can be expressed in terms of pOH. This is given by: pOH = -lg[OH^-]

It is possible to write an expression relating pH and pOH as pKw = pH + pOH

At 25^oC: pH + pOH = 14

Calculate the pH value of 0.001 moldm^-3 solution of NaOH at 25^oC.

[OH^-] = 10^-3

Therefore: p[OH^-] = 3

If pH + 3 = 14, then pH = 11

Calculate the pH of a solution of 0.1M hydrochloric acid at 25^oC:

[H⁺] = 10^-1 moldm^-3

pH = -log10^-1 = 1